Step index silica/glass as well as plastic optical fibers are generally composed of two or more materials, but primarily composed of a core having a higher refractive index than an outer, transparent, lower refractive index cladding material. This low refractive index cladding material improves the light carrying ability or efficiency of the fiber by preventing the escape of light from the core. The larger the difference in the refractive index between the core and the outer coating the greater the luminous properties of the fiber.
A measure of this light trapping efficiency is called the "Numerical Aperture", NA of a coated optical fiber. The angle at which a light ray may enter a fiber core and be propagated down the fiber without penetrating the surface of the core is termed the critical angle. This angle, and the fiber/cladding light trapping efficiency may be calculated from the respective refractive indices of the core and the cladding material as (n.sup.2.sub.1 -n.sup.2.sub.2).sup.1/2 =NA, where n.sub.1 and n.sub.2 are the refractive indices of the core and cladding respectively and NA is the Numerical Aperture. The Sine.sup.-1 NA is the critical angle, A.sub.c, and the angle of the cone of light that may enter a fiber without penetrating the surface of the core is the acceptance angle 2A.sub.c.
Thus, the larger the difference in the refractive index of the core versus the cladding material the greater the light gathering and trapping efficiency of the optical fiber.
In arriving at a suitable polymer for cladding an optical fiber it is readily apparent, from a summation of the atomic contribution to molar refraction of organic compounds that polymers containing fluorine, in place of hydrogen, would yield the lowest attainable refractive index. For example, fluorine has a molar refraction of 0.81 and hydrogen 1.028, see "Handbook of Chemistry and Physics, Chemical Rubber Publishing Co." From this type of calculation one would assume that commercially available polyfluorovinyl polymers would be the polymers of choice for low refractive index cladding materials. However, it has been found that the commercially available fluoro-polymers in general have high scattering losses. These losses are generally attributable to polymer crystallinity. For example, semi-crystalline fluoropolymers are typically translucent to opaque solids with scattering losses near 10.sup.6 dB/km. (L. Blyer Et. al. "Optical Fiber Telecommunications". E. S. Miller and A. Chynoweth, Academic Press, Inc. New York, 1979, 300-339).
Although numerous fluorine-containing polymer systems have been reported and/or patented as low refractive index cladding polymers the predominant, and most useful polymers to date are acrylate and methacrylate esters of fluorine-containing alcohols.
A secondary, but very important, purpose served by a cladding composition is to act as a protective coating on the surface of the optical fiber core to prevent silica-silica fiber abrasion. This is especially true for pure silica fibers which deteriorate rapidly when exposed to atmospheric moisture after being drawn from molten silica. It is important then to provide a protective cladding, which is most conveniently done with polymeric materials. In addition to lowering the refractive index, incorporation of fluorine into a cladding polymer structure also imparts an additional desirable, moisture barrier property, (L. Klinger; J. Mater. Res 2(6), Nov./Dec. 1987).
Because of the low viscosity and high volatility of the lower molecular weight fluorocarbon acrylate or methacrylate monomers, under normal circumstances, they cannot be conveniently used directly for cladding purposes. As a consequence, higher boiling acrylate esters of polyfluoro alcohols are currently used, such as in the Skutnik U.S. Pat. No. 4,511,209, where the higher boiling, 1H, 1H, 11H-eicosafluoroundecylacrylate, boiling at .about.290.degree. C. at atmospheric pressure, is used in UV curable cladding compositions. Because of the low Tg of the corresponding homopolymer a considerable amount of a trifunctional cross linking monomer is required to obtain a rigid polymer. Trimethylolpropyltriacrylate is recommended, at from 5.7 to 26.7%, in order to produce a hard cladding polymer, as claimed in the patent. The polymer containing 26.7% cross linker had a NA of 0.2 when polymerized on a quartz fiber. Comparatively, in this current invention, pure cladding polymers and copolymers can be produced from even the lowest boiling monomers, such as 2,2,2-trifluoroethyl acrylate, bp.about.100.degree. C. at atmospheric pressure and 2,2,2-trifluoroethyl methacrylate, bp.about.112.degree. C. at atmospheric pressure and the Polymer has a refractive index of 1.418 with a corresponding NA on a quartz fiber of 0.34.
U.S. Pat. No. 5,024,507, Jun. 18, 1991, to R. Minns, describes the use of copolymers of vinylidene fluoride and hexafluoropropylene (Fluorel, from 3M Co., U.S. Pat. No. 2,968,649, Jan. 16, 1961) and ter-polymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene, also from 3M, as viscosity modifying polymers. These polymers are apparently soluble in the cladding composition of fluorocarbon acrylate and diacrylate monomers used in their UV curable cladding solutions.
In the above mentioned Minns' patent and similar to the Skutnik patent the composition is composed of a 10% concentration of a cross linking monomer of a diacrylate ester of a long chain fluorocarbon diol, having low volatility, obtainable from the 3M Co. The resulting polymer is still elastomeric. A need exists for a simple low cost process for making optical fiber cladding solutions with an easily adjustable viscosity and refractive index.